A lightguide (otherwise known as a waveguide) traps light by providing total internal reflection with the lightguide. This is achieved by controlling the angles over which light enters the lightguide as well as selection of a material with suitable refractive index, which must be higher than the refractive index of the surrounding material, typically air.
Edge lit lightguides for back lighting and front lighting of displays are well known, and they are inexpensive and robust.
Known edge lit lightguides are designed to provide maximum uniformity of light output across the entire surface of the lightguide.
FIG. 1 shows a schematic image of an edge lit lightguide 10. The lightguide comprises a waveguide material, such as a slab of solid material with a top face 10a, a bottom face 10b and lateral edges 10c. There are top and bottom edges which cannot be seen in FIG. 1, as the cross section of FIG. 1 is taken in the lateral side-to-side direction. The lightguide is generally rectangular in plan view.
From the left side in FIG. 1, light is coupled in from a light source 12 and at the bottom of the lightguide several light out-coupling (i.e. light extraction) structures 14 (are placed. Light propagates under an angle θin inside the lightguide with height H. The out-coupling structures 14 in this example are drawn as half prisms with a half top angle α, height h, and a width w.
The lightguide is formed as a dielectric slab made out of e.g. glass or polycarbonate. In the slab, total internal reflection at the borders keeps the light confined while the light propagates. The edges of the slab are typically used to couple in light and the small light out-coupling structures 14 locally couple light out of the lightguide. The light extraction is achieved when the light hits a surface with scattering properties, or else the light hits a surface that breaks the parallelism that allowed the light to be reflected with angles larger than the critical angle.
In addition to LCD backlighting, lightguides are now also being used in office luminaires as well as in outdoor lighting. These new applications have also meant a departure from the flat slab based lightguides used in display applications.
For example, a cylindrical lightguide has been proposed in EP 2161599, in which light is emitted from a top annular surface of the cylinder. A lampshade with a tapered lightguide has been proposed in US 20130201717, in which the lampshade has a substantially cylindrical shape (that may be elliptic, parabolic or hyperbolic) with light extraction features applied to the inner surface.
Making an asymmetric light distribution (for example a lateral distribution Type III according to the classification of the Illuminating Engineering Society of North America (IESNA)) is a challenge using lightguides. Architectures using a circular cylindrical lightguide create rotationally symmetric patterns on a plane perpendicular to the lightguide. Due to the symmetry of the system, rays contained in the meridian planes (those containing the optical/symmetry axis) are controlled by the two-dimensional shape profile of the guide. The other non-meridian rays will also contribute to the rotationally symmetric pattern, although they are not controlled by the design.
In order to create an asymmetric light distribution with a circular cylindrical lightguide, it is possible to change the LED arrangement, in particular concentrating LEDs in some zones and/or diluting the density of the LEDs in other ones. Alternatively, the asymmetry may be achieved by dimming of some LEDs. This latter approach obviously limits the luminous flux that can be generated, because some LEDs are switched off or dimmed. There is also a disadvantage that this results in a very non-uniform appearance to the lightguide, which is not preferred in many applications.